Menthols, because of their desirable minty flavor and pleasant cooling effect, have been widely used as flavor additives for a variety of consumer products such as tobacco products, gums, medicinal products, toothpaste, foodstuffs, creams and lotions. Menthol is a particular aglycone which be derived from monoterpene glycosides, found in essential oil-containing plants. Such monoterpene glycosides function in the biosynthesis of terpenes as reactants or in the accumulation of essential oils as transport derivatives of free monoterpenes. Many glycosides of monoterpene alcohols are found in nature. See E. Stahl-Bishop, "Monoterpene Glycosides, State-of-the-Art", Flavor and Fragrance Journal, 2:75-82 (1987). The isolation and synthesis of menthols with the aim of developing menthol derivatives having good solubility in water has been described in I. Sakata and H. Iwamura, "Synthesis and Properties of Menthyl Glycosides", Agric. Biol. Chem., 43:307-312 (1979). These menthyl glycosides were determined to release menthol upon pyrolysis or hydrolysis.
Menthol is extensively used in tobacco products because of the refreshing cooling effect it imparts to tobacco smoke. Unfortunately, the high degree of volatility and ease of sublimation of menthol have presented problems in various manufacturing operations, such as packaging and handling, and in addition have resulted in a decreased shelf-life of the menthol-containing product due to losses of menthol by diffusion during storage. This problem is especially acute for menthol flavored filter cigarettes. During the processing and storage of this type of product, volatile menthol flavorants migrate from the tobacco and are irreversibly bound by the active adsorbents in the filter or the packaging. The menthol flavorant is therefore depleted and the effectiveness of the active adsorbent in the filter in selectively removing undesirable smoke components may be altered as well.
There has accordingly been considerable research aimed at developing methods of containing and preserving the menthol until it is used. Attempts to adsorb menthol on a support, such as charcoal or diatomaceous earth, and then adding the resultant composition to the tobacco have not been satisfactory. The menthol yields from such adsorbents are typically very low. Furthermore, this technique necessarily involves the incorporation of the adsorbent into the tobacco. The presence of the adsorbent in the tobacco often gives an undesirable appearance to the tobacco and may result in uneven burning of the tobacco. Efforts to contain menthol by microencapsulation in polymeric materials have also been largely unsuccessful.
In order to overcome these difficulties, research in this area has for many years been directed at preparing menthol derivatives which release menthol upon pyrolysis or hydrolysis. Various ester and carbonate derivatives of menthol which function in this manner are known.
For example, U.S. Pat. No. 3,312,226 reports that the ester compound 1-menthyl linalool carbonate, upon addition to smoking tobacco compositions, releases menthol during smoking due to the pyrolysis of the carbonate ester. However, such simple carbonate esters are not entirely satisfactory. Like menthol, they tend to migrate in the tobacco and accordingly hinder the quantitative release of menthol to the tobacco during smoking. Moreover, the second alcohol of the carbonate ester may be chemically altered during pyrolysis, resulting in the production of undesirable chemical fragments which impart a chemical aftertaste to the product.
U.S. Pat. No. 3,332,428 and U.S. Pat. No. 3,419,543 also utilize a carbonate ester to bind the menthol to a releasing agent. The smoking tobacco compositions of these patents, however, contain a menthyl carbonate ester of a polyhydroxy compound, such as a monosaccharide, disaccharide, trisaccharide, or glycol, which under smoking conditions, decomposes to release free menthol into the mainstream smoke. However, because these saccharides and glycols have only primary or secondary alcohol linkages, free menthol is produced upon pyrolysis with only limited efficiency.
U.S. Pat. Nos. 4,119,106, 4,092,988, 4,171,702, 4,177,339, and 4,212,310 describe the attaching of the menthyl carbonate ester derivative to other oligomeric and polymeric materials. Compositions containing these materials release volatile flavorants, including menthol, upon pyrolysis which occurs during the smoking of the tobacco compositions.
U.S. Pat. Nos. 4,532,944 and 4,578,486 describe smoking compositions which contain dicarbonate este compounds as flavoring additives. These compounds pyrolyze into menthol and other products which enhance the flavor of the mainstream smoke and the aroma of sidestream smoke.
U S. Pat. No. 4,540,004 discloses smoking compositions containing a 1,4-dioxane tetracarbonate derivitive. Under smoking conditions, this flavorant-release composition imparts an improved flavor to mainstream smoke and an improved aroma to sidestream smoke.
Japanese Pat. No. 2,283,992-A discloses certain glycosyl mono-menthyl orthoacetates which decompose in the presence of water or acid to liberate menthol. These compounds may be used as components for food and drinks.
The compounds and compositions disclosed in each of the references may yield a relatively small amount of available menthol, demonstrate a low level of stability, release other unwanted materials along with menthol, and cause difficulties in the preparation.
It is therefore an object of the present invention to develop compounds which release menthol upon pyrolysis or hydrolysis.
Another object of the invention is to provide a process for the preparation of the compounds of the present invention.
It is yet another object of this invention to provide a smoking composition which contains an effective amount of the compounds of the present invention in combination with tobacco or a tobacco substitute.